Protective Bonding Conductor Sizes

Sizes of main Bonding Conductors…

This can be confusing as the size of the main bonding conductor differs depending on the type of supply.

In a TN-S or TT system the main bonding conductor is in relation to the size of the main earthing conductor which is dependant on the size of the incoming line conductor.

e.g. A 25mm line supply conductor requires a 16mm main earth conductor (see table 54.7 BS 7671). The main bonding conductor in a TN-S or TT supply system must be at least half the cross sectional area of the main earth conductor (see 544.1.1 BS 7671).

Half the size of 16mm is 8mm but there’s no such cable size so we’ll go up to the next available size, which is 10mm for the main bonding conductor.

In a TN-C-S the main bonding conductor size is dependant on the size of the incoming neutral conductor (see 544.1.1 second paragraph and Table 54.8 BS 7671).

e.g. A TN-C-S supply has a 25mm neutral conductor, which is less than 35mm (Table 54.8) so a 10mm main bonding conductor is required.

Either way it’s 10mm.



Domestic Consumer Units

Consumer units in new and modern domestic installations should contain Residual Current Devices ‘RCDs’ as well as circuit breakers. Either as combined RCD/circuit breaker devices ‘RCBOs’ which can get a bit expensive or alternatively and less expensive through the use of split load boards with usually two RCCBs protecting multiple circuit breakers.

Since January 2016 and the introduction of Amendment 3 to the 17th Edition of BS7671…

All domestic consumer units must be made of a non-combustible material or enclosed in a cabinet made of a non-combustible material.

The layout of a split load consumer unit – the supply enters the unit via the double pole Main Switch which can be used to isolate the consumer unit/installation. From the out going side of main switch we have a Line and Neutral supply to the incoming side of each RCCB which in turn connects to the neutral bars and supplies the individual circuit breakers via the copper busbar at the bottom.

Things to note. The current rating on the Main Switch usually 100A is not the ‘tripping current’ of the device – as it offers no overload protection. The rating represents the safe switching current of the device only. Also on the RCBOs the current rating of usually 80A or 63A is also just the switching capacity of the device and the 30mA is the ‘tripping’ current of this RCD device.

Completed cosumer units should also have any blanks in place where needed and there should be no holes in the front, bottom or sides that are big enough to stick a finger in. On the top of the consumer unit there should be no holes larger than 1mm in diameter after completion. Stickers informing of the RCD test procedure should be in place as should any inspection and testing notices.

PVC Cables Concealed in Walls

The most common cable found within a domestic electrical installation is the PVC insulated and sheathed Twin and CPC cable – also known as Twin and Earth. A PVC three core and CPC is also used on two-way lighting circuits, smoke detector interlinks, fans with timers and also central heating thermostat connections etc.

Cables should always run within prescribed ‘cable zones’ which are 150mm down from the ceiling, 150mm at each side of any corners within a room and horizontally or vertically to any point or accessory.

When run below the surface of the wall PVC cables won’t react chemically with the plaster so there is no need to provide any protection unless it’s required mechanically (steel conduit, trunking, capping…)

If no mechanical protection is used and the cables are less than 50mm from the surface of the wall, which is usually the case then additional protection with a 30mA RCD must be used – see Regulation 522.6.202.


What’s an Electrician?


What makes an electrician?

The Electricity at Work Regulations 1989 (EAWR) – Regulation 16 states that…

“No person shall be engaged in any work activity where technical knowledge or experience is necessary to prevent danger or where appropriate injury, unless he/she possesses such knowledge or experience, or is under such a degree of supervision as may be appropriate having regard to the nature of the work.”

In other words you should not be doing any electrical work unless you know how to do it safely. If you don’t know how to do it safely then you should be supervised by someone who does.

BS 7671 gives guidance on how to comply with the Electricity at Work Regulations and helps with Regulation 16 by providing the following three definitions…

Skilled Person (electrically): Person who possesses, as appropriate to the nature of the electrical work to be undertaken, adequate education, training and practical skills, and who is able to perceive risks and to avoid hazards which electricity can create.

Instructed Person (electrically): Person adequately advised or supervised by a “skilled person” to enable that person to perceive risks and to avoid hazards which electricity can create.

Ordinary Person: Person who is neither a skilled person or an instructed person.

As you can see from the definition of a skilled person that experience alone is not proof of competency. Experience needs to be combined with the appropriate qualification(s) as evidence of having the technical knowledge and the practical skills required in order to recognise the risks and avoid the hazards involved when working with electricity.

For example, a domestic electrician may not have sufficient experience to work safely in a heavy industrial environment as an industrial electrician, and vice a versa but they may have the same qualifications ie C&G 2357/2365 17th Edition and Inspection and Testing (Initial and Periodic) etc.

“Electrician is a vague term for a person who complies through the guidance of good old BS 7671 with the Electricity at Work Regulations 1989 and has the qualifications and experience to demonstrate that they are a ‘Skilled Person’.”

NOTE: The Electricity at Work Regulations 1989 is a statutory document (ie the Law) and you can be prosecuted, leading to fines or even imprisonment for non compliance.

Electric Shocks

How an electric shock occurs…

Current travels through conductors within a closed circuit. The conductors could be made of copper such as in a cable, other metals, water or in the worst case the conductor can become the human body.

An electric shock occurs when a person becomes part of a circuit, with current entering the body at one point and leaving it at another.

The two methods of receiving an electric shock are…

Coming into contact with a normally live part (i.e. touching the live terminals within a consumer unit) or by touching a non-live part that has become live due to a fault in an electrical installation such as the case of a washing machine that has become live due to an earth fault

How serious the shock you receive depends on your physical condition, and general health.

The amount of current present – it’s the current that causes the problem and not the voltage.

And the path the current flows through your body and the length of time exposed to the current present.

So, remember, Voltage hurts but Current can kill

Non-lethal electric shocks occur when…

More than 3 milliamps (0.003A): You’ll receive a painful shock
More than 10 milliamps (0.01A): You’d feel muscle contraction
More than 20 milliamps (0.02A): This would really hurt and is considered severe electric shock
More than 30 milliamps (0.03A): You’d receive lung paralysis – usually temporary

Lethal electric shocks are…

More than 50 milliamps (which is only 0.05A): Possible ventricular fibrillation ‘heart flutter’ which is usually fatal
100 milliamps (0.1A) to 4 amps: Certain ventricular fibrillation and definitely fatal
Over 4 amps: Heart paralysis / severe burns and very fatal – electric chair!

If you see someone receiving an electric shock
Isolate the person from the electrical source. Turn off the electricity if possible, or move the person using a non-conducting material such as wood.
Never touch the person receiving an electric shock or you too could receive one.

Other first aid will quite likely be needed

The 17th Edition Has Still Got Legs

The 17th Edition has still got legs…

Although the 17th Edition of BS 7671 is due to be replaced by the 18th Edition in January 2019, the 17th Edition is relevant today and will still be as relevant long after that date.

The main reason for this is…

Any installation designed before January 2019 would only need to comply with the 17th Edition. Meaning, you could be installing to the 17th Edition well into the 2020s and even beyond.

And for anyone already with the 17th Edition including Amendment 3…

There’s usually an overlap period, as electricians are not expected to drop everything and head for the exam centres on the 1st of January 2019.
Plus, going off experience, the City & Guilds 2382-19? exam won’t be available till well after that date.

Qualified Supervisors, you’d probably be expected to update sometime during 2019 and within 2020 at the latest. The same goes for small business owners and JIB electricians – agency sparks, subcontractors etc. ‘Don’t forget to update your JIB card’.

It’s easier…

What I mean is, I’ve recently been going through the draft copy of BS7671 18th Edition and apart from it having around two hundred and fifty more pages (50% bigger). There’s plenty of new regulations, including a whole new part, ‘Part 8 – Energy Efficiency’.

“I won’t bore you with all the details of the new book for now, I’ll save that for a future post.”

This means that they are not just going to have to rewrite the whole exam for the 18th Edition, mainly to include the addition of Part 8, but the whole exam will have to be reconfigured.

This could mean a re-jig of the current exam with fewer questions on say Parts 4 and 5 or even the possibility of additional questions, raising the number of exam questions to above the current sixty.

The 18th Edition exam will be based on a bigger book with more pages to reference than the 17th Edition and in all likelihood, there will be more questions too.

However, there’s usually an update exam which will be opportunity for people who already have the 17th Edition up to Amendment 3 to take a shorter exam that’s just based on the changes introduced to the 18th Edition of BS 7671

In my opinion, passing the 17th and then the passing the 18th Edition update exam will be easier than completing the whole of the 18th Edition exam in one go.
So, the 17th Edition will be relevant well after January 2019, and passing it before then will make it easier to get your 18th Edition when the time comes.

Competent Person’s Schemes

In order to be able to sign off your own work under Part P you would need to be a member of a domestic installers competent person’s scheme ie with NAPIT, Elecsa, NICEIC etc.

To join a competent persons scheme you would need to demonstrate to them that you have sufficient knowledge and experience to complete your work safely and in compliance with any applicable regulations. The basic qualifications that could be used as evidence of knowledge are the 17th Edition Wiring Regulations C&G 2382, the Building Regulations C&G 2393 and an Inspection and Testing qualification such as the C&G 2392 ‘Initial Verification and Certification’.

These combined with evidence of experience such as references as well as work you have completed that they can examine could be enough to secure your place on a competent person’s scheme. Although installers with little experience may be required to complete a more formal – apprentice type course and qualification such as the old C&G 2365 up to at least level 2.

Our online courses at will provide the qualifications for the Building Regulations C&G 2393 and the Inspection and Testing C&G 2392 and our online workshop contains the domestic electrical installation theory training needed to be a good competent domestic installer.

We also provide the online training and the C&G 2382 exam for the 17th Edition Wiring Regulations at our online training website:

If you ring round the competent person’s scheme providers – the NICEIC may require more experience, but Elecsa and NAPIT are usually very helpful.

Approved Document P – Expained.

What is an Approved Document?

Approved documents are a series of documents that have been approved by the government and give practical ‘non-statutory’ guidance on how to meet the requirements of the Building Regulations which are a statutory (legal requirement).

If you follow the guidance within an approved document there is a presumption that you will comply with the requirements of the building regulations that it applies to.

“In plain language: If you comply with what is written in Approved Document P then you should comply with Building Regulation Part P.”

Part P of the Building Regulations came into effect in April 2013 and is for use in England.
Scotland come under the Building (Scotland) Act 2003 and Wales at this time are working to Approved Document P – 2006 updated to 2010.

There’s more information regarding Part P over at our new website 

‘Part P Trained’ is online training and seven day support for the Part P and the City & Guilds 2393 Building Regulations course and includes our very own domestic electrical installer’s online Workshops and offline training and is available today.


Installing a Shower – Electrically !

Wiring a Shower.

Perhaps one of the easiest circuits to install is the one that feeds a shower. Although it’s just a simple radial circuit there are a few pit holes you’ll need to avoid. We are going to assume that we are dealing with a domestic installation and as such Volt Drop will not be an issue, unless the shower is on the 3rd floor over on the east wing.

Size of cable required?

More than likely it will be a 10kW shower these days and it will use 10000/230 = 44A. So, we’ll need a cable that can carry 44A or more.

Looking at Table 4D5 BS 7671 we see that if we have the cable clipped direct (ref method C) we could use a 6mm sq twin and earth as it will carry up to 47A

However, it’s not a simple as that and it’s also highly unlikely that it’s going to be clipped directly throughout the building and we must allow for that.

Taking the scenic route and avoiding too much thermal insulation we’ll settle for reference method 100 and this will give us 45A current carrying capacity for a 10mm sq twin and earth.

Size of circuit breaker?

Have a look at Table 41.3 in BS761, not for the maximum Zs values for these circuits but for the size of circuit breakers commonly available.

You’ll notice that there is a 40A circuit breaker but, that would be too small as every time the shower was switched on it would trip. The next one shown is a 50A circuit breaker, but again this is no good as it is higher than the current carrying capacity of the 10mm sq cable (45A).

We can do one of two things.

Put a bigger cable in with a 50A circuit breaker – 16mm will carry up to 57A ref method 100. Good luck getting that in the pull switch though. Or…

Use a 45A circuit breaker! This will protect the cable against overload and they are available just not listed in BS7671. They are listed in the On-Site Guide, Table B6 along with the maximum Zs (80%) value.

So, it’s 10 mm sq cable and a 45A Type B BSEN 60898 circuit breaker.

But there’s more. According to Part 7 Section 701 BS7671 (701.411.3.3) we are also going to need Additional Protection using a 30mA RCD.

Other things to consider…

  • Is the supply big enough for the additional load?
  • Will you need to put a separate consumer unit (shower unit) at the distribution board or will it fit into the existing one?
  • Is all of the earthing and bonding satisfactory?
  • Position of the pull switch – at least 600mm away from the edge of the bath or shower cubical if the ceiling is less than 2.25m.

Installing a shower can easily escalate into a ‘big job’ especially if you need to add an additional consumer unit and upgrade all of the earthing and bonding.

Plus don’t forget to inspect and test and it’ll need a Electrical Installation Certificate and also ‘notifying ‘ under Part P as well as it’s a new circuit and also because it’s in the bathroom.

See my other blog posts for max Zs value, types of circuit breakers, additional protection, earthing and bonding etc and also BS7671 for more information if in doubt.

There’s also more information regarding training Part P of the Building Regulations over at our new City & Guilds Part P course website.







Isolation and Switching Chapter 53 BS 7671

Key Points of…

Isolation: 537.2

Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributors incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices.

Switching off for mechanical maintenance: 537.3

Mechanical maintenance is work that does not involve exposure to electrical connections. As such this work is often undertaken by ‘ordinary persons’.

Examples: Cleaning, adjusting or replacing parts of a machine. Replacing lamps in a fluorescent light.

Isolating devices must be able to switch the full load current and should be local to the equipment. More often than not the main isolating device is also used for mechanical maintenance for machinery as it’s close by and lockable. Other devices may be used such as, fused connection units, double pole switches and plugs and sockets.

Emergency switching: 537.4

Emergency switching should be provided for any part of an installation where it is necessary to isolate the supply quickly to remove unexpected danger.

In installations with rotating machinery this is usually done by using ‘stop’ buttons. Care should be taken to ensure that the stop buttons are…

  • Near to the machine / equipment
  • Should not be capable of being reset remotely, unless with a key switch.
  • If remote resetting is possible then the activated stop button must remain activated at the point of the danger until being reset once the danger is removed.
  • In areas where there are untrained people the stop buttons should lock when operated.

Functional switching: 537.5

The most common type of switching found in an installation. For general use to control any part of an installation that may need to be controlled independently from other parts.

They should be conveniently located and easily operated manually if needed. They include…

  • Light switches
  • Plugs and sockets (below or at 32A)
  • Switched fused spares
  • PIR sensors
  • Time clocks / thermostats etc

Functional switches may not include…

  • Fuses
  • Luminaire connections – plug in type ‘Clix’
  • Unswitched fused spurs
  • Socket outlets above 32A

“These are just the key points. For more detailed information see good old BS 7671 Part 5 Selection and Erection: Chapter 53 Protection, Isolation, Switching, Control and Monitoring”